Partial Pressures: Solved Problems for Exam Preparation

📚 Quick Study Guide

• ⚛️ Dalton's Law of Partial Pressures states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases: $P_{total} = P_1 + P_2 + ... + P_n$.
• 🌡️ The partial pressure of a gas is the pressure that the gas would exert if it occupied the same volume alone: $P_i = X_i * P_{total}$, where $X_i$ is the mole fraction of gas $i$.
• ⚗️ The mole fraction of a gas is the ratio of the number of moles of that gas to the total number of moles of all gases in the mixture: $X_i = \frac{n_i}{n_{total}}$.
• 📐 Remember to use consistent units for pressure (e.g., atm, kPa, mmHg) and temperature (Kelvin) in your calculations. Convert to Kelvin by adding 273.15 to Celsius: $T(K) = T(°C) + 273.15$.
• 💡 When collecting a gas over water, remember to subtract the vapor pressure of water from the total pressure to find the partial pressure of the gas: $P_{gas} = P_{total} - P_{H_2O}$.

🧪 Practice Quiz

1. What is the partial pressure of oxygen in a mixture containing 1 mole of oxygen, 2 moles of nitrogen, and 3 moles of helium, if the total pressure is 2 atm?
   1. 0.33 atm
   2. 0.67 atm
   3. 1.0 atm
   4. 1.33 atm
2. A container holds 4g of Helium and 16g of Oxygen. What is the mole fraction of Helium?
   1. 0.2
   2. 0.5
   3. 0.8
   4. 0.9
3. A gas mixture contains nitrogen, oxygen, and carbon dioxide. The partial pressure of nitrogen is 20 kPa, oxygen is 40 kPa, and the total pressure is 100 kPa. What is the partial pressure of carbon dioxide?
   1. 20 kPa
   2. 30 kPa
   3. 40 kPa
   4. 60 kPa
4. If the partial pressure of water vapor in a gas collected over water at 25°C is 3.2 kPa, and the total pressure is 101.3 kPa, what is the partial pressure of the dry gas?
   1. 95.8 kPa
   2. 98.1 kPa
   3. 104.5 kPa
   4. 101.3 kPa
5. A container has 2 moles of gas A and 3 moles of gas B. If the total pressure is 5 atm, what is the partial pressure of gas A?
   1. 1 atm
   2. 2 atm
   3. 3 atm
   4. 4 atm
6. At 27°C, gas is collected over water. The total pressure is 770 mmHg. The vapor pressure of water at 27°C is 27 mmHg. What is the partial pressure of the dry gas?
   1. 743 mmHg
   2. 797 mmHg
   3. 770 mmHg
   4. 27 mmHg
7. Which of the following statements is true regarding Dalton's Law of Partial Pressures?
   1. The total pressure is less than the sum of partial pressures.
   2. The total pressure is equal to the product of partial pressures.
   3. The total pressure is equal to the sum of partial pressures.
   4. The total pressure is equal to the average of partial pressures.